feat(hub): add Downloads and Documentation landing pages

Wire hub-downloads.js for stable OTA APK resolution, expand nav i18n, and add
Session Studio download slot for alpha hub FR.

Co-authored-by: Cursor <cursoragent@cursor.com>

hub-landing.css

@@ -474,3 +474,59 @@
   .landing-scroll { scroll-behavior: auto; }
   .landing-rail { transition: none; }
 }
+
+/* Downloads / documentation big rounded CTAs */
+.landing-download-grid {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 16px;
+  margin: 20px 0 12px;
+}
+
+.landing-download-btn {
+  min-width: 200px;
+  min-height: 56px;
+  padding: 16px 28px;
+  border-radius: 999px;
+  font-size: 1.05rem;
+  font-weight: 600;
+  text-align: center;
+  text-decoration: none;
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  transition: box-shadow 0.2s ease, transform 0.15s ease;
+}
+
+.landing-download-btn:hover,
+.landing-download-btn:focus-visible {
+  transform: translateY(-1px);
+  box-shadow: 0 0 30px rgba(61, 139, 253, 0.45);
+}
+
+.landing-download-hint {
+  margin-top: 8px;
+  font-size: 0.92rem;
+}
+
+.landing-docs-grid {
+  display: grid;
+  grid-template-columns: repeat(auto-fit, minmax(220px, 1fr));
+  gap: 16px;
+  margin-top: 16px;
+}
+
+.landing-docs-card h3 {
+  margin: 0 0 10px;
+  font-size: 1rem;
+}
+
+.landing-docs-list {
+  margin: 0;
+  padding-left: 1.1rem;
+  line-height: 1.65;
+}
+
+.landing-docs-list a {
+  color: var(--accent, #3d8bfd);
+}